Unraveling Depth-Specific Ionic Conduction and Stiffness Behavior across Ionomer Thin Films and Bulk Membranes

Abstract

Interfacial behavior of submicron thick polymer films critically controls the performance of electrochemical devices. We developed a robust, everyday-accessible, fluorescence confocal laser scanning microscopy (CLSM)-based strategy that can probe the distribution of mobility, ion conduction, and other properties across ionomer samples. When fluorescent photoacid probe 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (HPTS) was incorporated into <1 μm thick Nafion films on substrates, the depth-profile images showed thickness- and interface-dependent proton conduction behavior. In these films, proton conduction was weak over a region next to substrate interface, then gradually increased until air interface at 88% RH. Conversely, consistently high proton conduction with no interface dependence was observed across 35–50 μm thick bulk, free-standing Nafion membranes. A hump-like mobility/stiffness distribution was observed across Nafion films containing mobility-sensitive probe (9-(2-carboxy-2-cyanovinyl)­julolidine) (CCVJ). The proton conduction and mobility distribution were rationalized as a combinatorial effect of interfacial interaction, ionomer chain orientation, chain density, and ionic domain characteristics.

Molecular-Level Control over Ionic Conduction and Ionic Current Direction by Designing Macrocycle-Based Ionomers

“…The films we studied had thickness comparable to that of the ionomer-based binder layers over catalyst particles in hydrogen fuel cells and electrolyzers. Especially, such improvement in proton conductivity at low % RH stands out 97 as most of the reported ionomers (both fluorocarbon- and hydrocarbon-based) showed poor proton conductivity at low % RH in both bulk membranes 98 − 100 and the thin film 7 , 8 , 101 format.…”

“…The confocal laser scanning microscopy (CLSM) imaging of a Nafion–calix-2 composite film ( Figure 6 ) provided a strong experimental evidence of the role of macrocyclic cavities on proton conduction. In our prior work, 7 we have demonstrated this CLSM-based strategy to reveal a through-plane ( z -direction) proton conduction profile across ionomer films. By now, we know that calix-2 forms ellipsoidal features, while Nafion makes featureless films (AFM images, Figure 4 ).…”

“…The films we studied had thickness comparable to that of the ionomer-based binder layers over catalyst particles in hydrogen fuel cells and electrolyzers. Especially, such improvement in proton conductivity at low % RH stands out 97 as most of the reported ionomers (both fluorocarbonand hydrocarbon-based) showed poor proton conductivity at low % RH in both bulk membranes 98−100 and the thin film 7,8,101 format. The EIS data above and our prior understanding of synthetic ion channels 34,42,54,60,62 suggested that the exceedingly high proton conductivity of calix-2 films originated from the sulfonated macrocyclic cavities.…”

“…Ionomers with high ionic conductivity and permselectivity are highly needed to address the technical challenges of energy conversion and storage devices (such as fuel cells, electrolyzers, and batteries). − Ionomers are used as several tens of micrometer-thick, bulk, free-standing membrane separator and a sub-micrometer-thick catalyst binding layer in these devices. However, hydrogen fuel cells experience a major ion transport limitation within the sub-micrometer-thick ionomer layer. − This slows down the electrocatalytic reaction at the cathode and affects the performance of the device. A ∼25 μm-thick bulk, free-standing membrane of the current state-of-the-art, fluorocarbon-based ionomer Nafion, conducts protons efficiently, while the proton conductivity of the same ionomer becomes about an order of magnitude lower in a ∼25 nm-thick film on the substrate. ,, The high conductivity in bulk membrane − and low conductivity in thin films/catalyst layers ,− have also been reported for polyaromatic ionomers.…”

“…We hypothesize that the aromatic ionomers having macrocycle-based repeat units along the chain can leverage multiscale ion transport pathways: (i) along the −SO 3 H groups of all units (macrocyclic and nonmacrocyclic) of an ionomer chain and (ii) across macrocyclic cavities. Even if the ionomer chains experience severe confinement and fail to spontaneously phase-segregate (as seen in sub-micrometer-thick Nafion films ,,, ), the individual macrocyclic repeat units of calix-2 chains can still maintain the through-cavity ionic conduction. On top of this, if we are able to achieve favorable self-assembly of pore-forming units in solution (likely based on prior evidences ,− ) and retain the self-assembled feature of the solid-state, we will be able to achieve both short-range and long-range ionic conduction simultaneously.…”